Parametric sound is a fundamentally new class of audio, which relies on a non-linear mixing of an audio signal with an ultrasonic carrier. One of the key enablers for this technology is a high-amplitude, efficient ultrasonic source, which is referred to here as an emitter or transducer. Ultrasonic emitters can be created through a variety of different fundamental mechanisms, such as piezoelectric, electrostatic, and thermoacoustic, to name a few. Electrostatic emitters are generally capacitive devices consisting of two conductive faces with an air gap, where at least one of the conductive faces has a texture that is critical to the functionality of the emitter.
Non-linear transduction results from the introduction of sufficiently intense, audio-modulated ultrasonic signals into an air column. Self-demodulation, or down-conversion, occurs along the air column resulting in the production of an audible acoustic signal. This process occurs because of the known physical principle that when two sound waves with different frequencies are radiated simultaneously in the same medium, a modulated waveform including the sum and difference of the two frequencies is produced by the non-linear (parametric) interaction of the two sound waves. When the two original sound waves are ultrasonic waves and the difference between them is selected to be an audio frequency, an audible sound can be generated by the parametric interaction.
Parametric audio reproduction systems produce sound through the heterodyning of two acoustic signals in a non-linear process that occurs in a medium such as air. The acoustic signals are typically in the ultrasound frequency range. The non-linearity of the medium results in acoustic signals produced by the medium that are the sum and difference of the acoustic signals. Thus, two ultrasound signals that are separated in frequency can result in a difference tone that is within the 60 Hz to 20,000 Hz range of human hearing.
Conventional audio systems have been implemented using electrostatic, or ‘push-pull’ audio speakers. FIG. 1 is a diagram illustrating a simple example of an electrostatic speaker. Conventional electrostatic audio speaker typically includes three basic components—stators 122, a diaphragm 112, and spacers 124. The stators 122 are typically made of insulator coated metal grids. The diaphragm 112 is a lightweight electrically conductive film stretched parallel to and between the two stators. For operation, the diaphragm 112 is charged to a fixed positive potential by a high-voltage power supply to create the charge. Once charged it can be forced to move by the application of an electric field between the stators.
The electric field is provided by applying large (1000+) differential voltages to the front and rear stators. The stators are connected to the system's audio amplifier and are charged by the voltage of the amplified audio signal. The voltage applied to one stator is equal or substantially equal to but the opposite polarity of the voltage applied to on the other stator. In response to the audio input signal, the voltage alternates between the stators 122 causing the diaphragm 112 to move in relation to the audio signal. This movement of the diaphragm 112 forces acoustic waves into the air. These waves are transmitted through both stators 122 and into the room. When the film is moving forward in each cycle, a positive pressure wave is emitted in the forward direction, and a refraction wave is emitted in the reverse. This positive plus negative pressure field is commonly called a ‘dipole’ speaker and increases directionality at the cost of low frequency reproduction. Because of this, electrostatic hi-fi speakers are almost always accompanied by a woofer to fill in the lower end of the frequency response.
There are some fundamental design considerations when engineering a traditional electrostatic. First, to achieve maximum fidelity (linear response), the film should be placed equidistant between the stators. Second, both stators 122 are open to prevent any trapped air from forming a mechanical resonance in the audio band of frequencies. The configuration is typically designed to deemphasize resonance to provide a relatively flat response across the range of operating frequencies. Second, because the stators 122 need to accommodate a significant movement of the diaphragm 112, they are positioned fairly far apart (˜1 cm or more for the ˜mm excursion of the film). Accordingly, the speaker has very low voltage sensitivity and needs 1000+V to achieve significant output. Lastly, the dipole response is a necessary consequence of this design and is not always preferred in a listening environment (compared to the monopole response of a traditional loudspeaker.